Acanthamoeba castellanii is a small, free living amoeba that is the causative agent for Acanthamoeba keratitis. Acanthamoeba can support intracellular growth for several human pathogens and may represent a significant environmental reservoir for bacteria, which require intracellular growth. Acanthamoeba is characterized by a mitotic growth cycle, and formation of an encapsulated cyst when unfavorable conditions are encountered. The molecular mechanisms of differentiation are largely unknown. This is proposal to characterize mechanisms of gene expression during growth and differentiation. Two genes essential for growth, the TBP gene and the gene of the factor, which regulates it, TPBF, will be characterized in vitro and in vivo. The gene encoding a transcriptionlly regulated, cyst specific protein (CSP21) has been cloned, providing an opportunity to characterize the mechanisms of cell-specific gene expression.